Space Radiation Cancer Risks and Uncertainties for Mars Missions

Abstract

Projecting cancer risks from exposure to space radiation is highly uncertain because of the absence of data for humans and because of the limited radiobiology data available for estimating late effects from the high-energy and charge (HZE) ions present in the galactic cosmic rays (GCR). Cancer risk projections involve many biological and physical factors, each of which has a differential range of uncertainty due to the lack of data and knowledge. We discuss an uncertainty assessment within the linear-additivity model using the approach of Monte Carlo sampling from subjective error distributions that represent the lack of knowledge in each factor to quantify the overall uncertainty in risk projections. Calculations are performed using the space radiation environment and transport codes for several Mars mission scenarios. This approach leads to estimates of the uncertainties in cancer risk projections of 400-600% for a Mars mission. The uncertainties in the quality factors are dominant. Using safety standards developed for low-Earth orbit, long-term space missions (>90 days) outside the Earth's magnetic field are currently unacceptable if the confidence levels in risk projections are considered. Because GCR exposures involve multiple particle or delta-ray tracks per cellular array, our results suggest that the shape of the dose response at low dose rates may be an additional uncertainty for estimating space radiation risks.

Heavy ion carcinogenesis and human space exploration

“…Risks for extended missions to the moon and the Mars exploration mission exceed 3% for most Mars mission scenarios with upper 95% confidence levels near 15% risk of death 9 . The scaling of mortality rates for space radiation risks to astronauts to the Atomic bomb survivors introduces many uncertainties 1,2,8 into risk estimates (Figure 1), and there are important questions with regard to the correctness of any scaling approach because of qualitative differences in the biological effects of HZE ions and γ-rays.…”

“…Furthermore, overall uncertainties in space radiation cancer projections are about 5-fold times higher at the 95% confidence level 8,9 than the median risk projection and it is not possible, with the current state of knowledge of cancer biology, to judge if risks are higher or lower than safety standards. These facts further support that improving the understanding of the biology of cancer risks from space radiation exposure is the primary hurdle for a Mars mission.…”

Evaluating shielding effectiveness for reducing space radiation cancer risks

“…In estimating cancer risks for space radiation, additional uncertainties occur related to estimating the biological effectiveness of protons and heavy ions, and predicting LET spectra at tissue sites (Cucinotta et al, 2001b). The limited understanding of heavy ion radiobiology has been estimated to be the largest contributor to the uncertainty for space radiation effects (NAS, 1996), and radiation quality factors found to contribute the major portion of the uncertainties in a previous study (Cucinotta et al, 2001b).…”

“…The final improvement made here is to accumulate Monte-Carlo trials directly from the double-detriment life-table rather than the cancer mortality rate. The life-table approach is used because in our previous estimates of cancer risk for the Mars mission (Cucinotta et al, 2001b), the upper level of the 95% C.I. exceeded 10% mortality risk, and for we expect that competing causes of death may significantly compress the higher values in the distribution.…”

Description of light ion production cross sections and fluxes on the Mars surface using the QMSFRG model

“…Prior to their arrival an accurate assessment of the radiation environment on the Mars surface is required. Particle flux distributions, which describe particle type (charge and mass), j, kinetic energy, E(MeV/u) and particle directions are necessary for a complete description of radiation fields in space because the use of dose equivalent carries large uncertainties in projecting risk of late effects from galactic cosmic rays (GCR) [1][2][3]. The primary GCR are well described by empirical models based on extensive measurements [4].…”